The present invention relates to electrical circuits, and more particularly to a circuit and method for transmitting digital-to-analog data over more than one channel employing a single DAC.
Current switching digital-to-analog converters (DACs) are utilized in a variety of applications such as instrumentation applications, level detection applications, drivers for LCD screens, servo tracking and communication applications. In a current switching digital-to-analog converter, inputed binary code turns on a selection of current sources wired to generate appropriately scaled currents. The output currents are summed and are either brought out directly as a current output or converted internally to a voltage via an op-amp summing junction circuit or the like. Recently DACs have been employed in greater numbers in communication devices to provide digital capabilities to devices, while providing analog signaling to communication lines. Typically in these implementations where multiple channels are involved, a single DAC is dedicated to each channel. These DACs take up substantial space when implemented on an integrated circuit device.
One communication technology that has evolved that employs DACs for providing digital-to-analog conversions over communication channels is known as the Asymmetric Digital Subscriber Line (ADSL). ADSL is a new modem technology that converts existing telephone lines into access paths for multimedia and high speed data communications. ADSL utilizes advanced digital signal processing and creative algorithms to squeeze greater amounts of information through twisted-pair telephone lines than was conventionally feasible. The ADSL standard calls for a specifically designed modem at each end of a twisted pair copper line, one at a home and the other at a central office of the phone company. Although the conventional telephone voice circuit has only a 4 khz bandwidth, the physical wire connection bandwidth is more than 1 MHz.
ADSL exploits the extra bandwidth to send data to the central office where it can connect to a phone company""s high capacity fiber optic network. ADSL modems increase the amount of information that conventional phone lines can carry buy using discrete multi-tone technology (DMT). DMT divides the bandwidth into independent sub-bands, then transmits data on all of the subbands simultaneously. Echo cancellation techniques allow upstream and downstream data to overlap. The standard also reserves a four KHz region at the DC end of the frequency band for POTS (i.e., Plain Old Phone System). ADSL is particularly attractive for consumer Internet applications where most of the data traffic is downloaded to the customer. Upstream bandwidth for uploading data can be reduced to increase downstream bandwidth since most Internet traffic is downstream.
Terminals employed at the central office typically communicate over several channels. These terminals employ modems or other communication devices for data transmissions. These modems or other communication devices can employ a digital processor, a coder/decoder component, line drivers and other peripheral devices to support transmitting and receiving of analog signal transmissions. The coder/decoder components include DACs and other devices to convert digital data to analog data for transmission over communication channels. These coder/decoder components can be in the form of integrated circuits, such that the DACs and other devices are integrated into a single chip. A DAC is required for each transmit channel that is provided by the coder/decoder component. The DAC consumes power and takes up more real estate on the integrated circuit than other components.
In view of the above, it is apparent that there is an unmet need for improvements in the utilization of DACs for various applications including communication systems.
According to one aspect of the present invention, a circuit and method is provided that allows for communication of digital-to-analog data over more than one channel employing a single current switching DAC and a current switching multiplexer. The current switching multiplexer is an output stage circuit that is used to steer the current from one output channel to another. The data rate of the data transmitted to the DAC is increased by the number of channels over which the data is transmitted. The data is then switched from one channel to the other by employing a current switching multiplexer, such that the device provides the same functionality of conventional devices which utilize a single DAC for each channel.
The current switching multiplexer includes a first and second differential current switch embedded in a differential inverter circuit. The differential inverter circuit includes a plurality of n-transistors and p-transistors that are sized to provide matching transconductances for smooth transition switching.
In one aspect of the invention, the current level of at least one unselected channel can be set to provide increased switching speed and better overall performance to current to voltage converters coupled to the unselected output channel. Typically, the current is set to a midpoint current very close to a transition point, such that settling time of a current to voltage converter (e.g., a low pass filter) is reduced. Controlling the midpoint current of the unselected output provides fast switching of data between channels and reduced harmonic distortion, thereby boosting performance of the data transmission circuit.
Thus, according to another aspect of the present invention, a multiple channel data transmission circuit is provided. The multiple channel transmission circuit comprises a current switching digital-to-analog converter operable to receive digital data and generate a pair of differential analog current signals corresponding to the digital data. A current switching multiplexer is coupled to the current switching digital-to-analog converter. The current switching multiplexer is operable to switch the pair of differential analog current signals between at least two channels. A current-to-voltage converter component is provided for each of the at least two channels. The current-to-voltage converter component is operable to convert the pair of differential analog current signals to a pair of differential analog voltage signals.
In accordance with another aspect of the present invention, an ADSL communication circuit is provided for transmitting data across multiple channels. The ADSL communication circuit comprises a current switching digital-to-analog converter operable to receive frames of digital data to be transmitted across a first and a second channel. The current switching digital-to-analog converter is adapted to generate sequences of differential analog current signals corresponding to the frames of digital data. A current switching multiplexer is coupled to the current switching digital-to-analog converter. The current switching multiplexer is operable to switch the sequences of differential analog current signals between a first and a second output based on a selection and a non-selection of a first and a second selection control line. A first current to voltage converter circuit is coupled to the first output of the current switching multiplexer and a second current to voltage converter is coupled to the second output of the current switching multiplexer. This current to voltage converter can be a current input low pass filter. In the detailed description, a low pass filter circuit will be utilized to indicate an exemplary current to voltage converter. The first low pass filter is adapted to convert sequences of analog current signals to sequences of analog voltage signals for transmission over the first channel and the second low pass filter is adapted to convert sequences of analog current signals to sequences of analog voltage signals for transmission over the second channel.
Yet another aspect of the invention relates to method for transmitting data over multiple communication lines. The method comprises the steps of providing frames of digital data assigned to more than one channel to a current switching digital-to-analog converter and converting the frames of digital data to sequences of analog current data by the current switching digital-to-analog converter. The sequences of analog current data are then switched between multiple channels based on a channel that the data is assigned. The analog current data is then converted to analog voltage data prior to transmission over a communication line.
In accordance with yet another aspect of the invention, an ADSL communication circuit is provided for transmitting data across multiple channels. The communication ADSL circuit comprises means for converting frames of digital data to be transmitted over at least two channels to sequences of analog current data, means for switching the sequences of analog current data between the at least two channels and means for selecting the channel of the at least two channels for transmitting the sequences of analog current data. The communication ADSL circuit also comprises means for converting the analog current data to analog voltage data prior to transmission of the data over a communication line.
To the accomplishment of the foregoing and related ends, the invention comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed and the present invention is intended to include all such embodiments and their equivalents. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.